java - 未应用 Hibernate @Fetch(Join) 而不是单独的 SELECT

Question

我有一个 Domain 实体，它与我希望立即加载的 Country 实体具有 @ManyToOne 单向关系使用 JOIN 而不是单独的选择。我考虑过使用@Fetch。

@Entity
@Table
public class Domain {
    @Id
    @GenericGenerator(name = "generator", strategy = "increment")
    @GeneratedValue(generator = "generator")
    @Column(name = "domain_id")
    private Long domainId;

    @Fetch(FetchMode.JOIN)
    @ManyToOne(optional = false, fetch = FetchType.EAGER)
    private Country country; 

    ...
}

我正在使用 HQL 来查询这些实体。

但是 Hibernate 不应用获取策略。我尝试过使用 @OneToOne (这不会改变我的设计中的任何内容)，但这也不起作用。这是 SQL 输出示例

Hibernate: select domain0_.domain_id as domain1_2_, domain0_.country as country2_, domain0_.name as name2_, domain0_.type as type2_ from domain domain0_ order by domain0_.name limit ?
Hibernate: select country0_.country_id as country1_1_0_, country0_.code as code1_0_, country0_.currency as currency1_0_, country0_.name as name1_0_ from country country0_ where country0_.country_id=?
Hibernate: select country0_.country_id as country1_1_0_, country0_.code as code1_0_, country0_.currency as currency1_0_, country0_.name as name1_0_ from country country0_ where country0_.country_id=?
Hibernate: select country0_.country_id as country1_1_0_, country0_.code as code1_0_, country0_.currency as currency1_0_, country0_.name as name1_0_ from country country0_ where country0_.country_id=?
Hibernate: select country0_.country_id as country1_1_0_, country0_.code as code1_0_, country0_.currency as currency1_0_, country0_.name as name1_0_ from country country0_ where country0_.country_id=?
Hibernate: select country0_.country_id as country1_1_0_, country0_.code as code1_0_, country0_.currency as currency1_0_, country0_.name as name1_0_ from country country0_ where country0_.country_id=?

我假设有什么东西阻止了它的应用。那是什么？

Answer 1

我正在查询我的 Domain 实体，例如

FROM Domain domain WHERE domain.type = :type

这导致 Hibernate 为返回的每个 Domain 单独查询 Country 实体。

基于汤姆·安德森的评论和答案here ，我将 HQL 更改为

FROM Domain domain JOIN FETCH domain.country WHERE domain.type = :type

这会导致 Hibernate 仅使用一个带有联接的大型查询来一起检索 Domain 和 Country，而不是对所有 Domain 使用一个查询code> 实体和用于检索每个实体的 Country 的 SELECT。

对于Criteria，似乎没有必要这样做。仅当您使用HQL时，才需要指定JOIN FETCH。